Field of the Invention
This invention relates to a process for manufacturing terephthalic acid and more particularly, to a process for manufacturing the highly purified terephthalic acid in a high recovery rate, in accordance with the practice of this invention comprising the following procedures: polyethylene terephthalate (hereinafter referred to as "PET") scrap is hydrolyzed with an akaline aqueous solution to give the slurry of terephthalic acid alkali metal/earth metal salt, then the particle of terephthalic acid formed from acid-neutralization is enlarged to recover the highly purified terephthalic acid.
The PET waste scrap materials, which are discharged after using them from the end-users, refer to polyester scrap textiles, PET scrap bottles, PET scrap containers, polyethylene terephthalate scrap chips generated in the polymerization process of PET, or polyester scrap yarns generated from the process of manufacturing polyester fibers yarns and textiles. The need for the reutilization of the PET scrap, being incessantly discharged by one's daily life or in the manufacturing process, has created a great issue in the viewpoint of reducing production costs and abating environmental problems. Recently, various processes have been proposed for the recovery of terephthalic acid from the PET scrap but they arenot efficient and economical.
The conventional processes of manufacturing and recovering terephthalic acid are as follows, using the PET scrap:
In U.S. Pat. Nos. 3,120,561 and 4,578,502, PET was hydrolyzed at high temperature and pressure, cooled rapidly and crystallized to recover the precipitated terephthalic acid.
In U.S. Pat. No. 3,884,850, bis(hydroxyethyl) terephthalate was employed as a solvent to recover terephthalic acid from PET.
In U.K Pat. No. 2,123,403 and Japanese Patent Unexamined Publication No. 3-16,328, PET was hydrolyzed with a solvent such as water in the presence of decoloring carbon at 200 to 300.degree. C., then cooled under reduced pressure to recover terephthalic acid.
In U.S. Pat. No. 3,952,053, two methods related the recovery of terephthalic acid from PET as follows: a) through the hydrolysis with sulfuric acid, the obtained mixing solution consisting of terephthalic acid and sulfuric acid was precipitated by water to recover terephthalic acid, or b) PET was placed in an aqueous solution of sodium hydroxide to precipitate unsoluble materials for removal and sulfuric acid was added to precipitate terephthalic acid. Then, ethylene glycol was extracted with organic solvent and distilled after the recovery of terephthalic acid.
In U.S. Pat. No. 4,355,175, PET was hydrolyzed by acid, diluted with cold water and filtered immediately. The resulting solution was dissolved in alkali hydroxide solution to precipitate impurities for removal and added with sulfuric acid to precipitate terephthalic acid. Then, the solution was filtered, washed with water and dried to recover terephthalic acid.
In U.S. Pat. No. 3,544,622, PET was reacted by saponification with sodium hydroxide solution at 150.degree. C. in the presence of ethylene glycol, to prepare disodium terephthalate. Then, the resulting solution was filtered, washed with ethylene glycol or aqueous solution of disodium terephthalate at over 90.degree. C. and dissolved in water. Activated charcoal was added to the solution at 90.degree. C., agitated and neutralized with sulfuric acid. Then, terephthalic acid was filtered and washed with water to recover terephthalic acid.
In European Patent No. 497,662, PET was reacted with alkali metal/earth metal hydroxide at atmospheric pressure and 140 to 180.degree. C., to prepare terephthalic acid alkali metal/earth metal salt. This material was dissolved in water to extract impurities with C.sub.3 to C.sub.8, alcohols, then neutralized with acid and followed by filtration to recover terephthalic acid.
In European Patent No. 597,751, PET was reacted with sodium hydroxide in the presence of a mixing extruder without the addition of solvent and then, the obtained disodium terephthalate was dissolved in water, passed through activated charcoal and neutralized with sulfuric acid. The resulting solution was filtered and washed to recover terephthalic acid.
In U.S. Pat. No. 5,395,858, PET dissolved in sodium hydroxide solution was heated to prepare both disodium terephthalate and ethylene glycol. These materials, so obtained, were heated up higher than boiling point of ethylene glycol to evaporate the solution. The remaining disodium terephthalate was dissolved in water and neutralized with acid to recover terephthalic acid.
These reported methods as aforementioned have also several problems as follows; a) a majority of their reactions was conducted at high temperature and pressure, b) they failed to illustrate some methods of removing impurities and monitoring purity, and c) even in case of monitoring said purity, the purity of terephthalic acid was not analyzed by appropriate methods for the quality measurements. In addition, in a process of filtering terephthalic acid as a final recovery step, the particle size of terephthalic acid should be sufficiently enlarged because small particles of terephthalic acid cause insufficient separation into solids and liquids which is responsible for reduction of recovery rate, and also make it difficult to perform the drying process. Nevertheless, said reported methods did not mention any steps of enlarging the particle sizes of terephthalic acid.
In this context, said reported methods are technically and economically unfavorable for commercialization and further, there are still plenty of rooms for improving environmental problems, since said reported methods failed to suggest the method of treating by-products generated in the recovery process.